1. Field of the Invention
The present invention relates to methods of discovering, diagnosing and correcting formation integrity problems in successively drilled subterranean well bore intervals.
2. Description of the Prior Art
In the drilling of wells (for example, oil and gas wells) using the rotary drilling method, drilling fluid is circulated through a drill string and drill bit and then back to the surface by way of the well bore being drilled. The drilling fluid maintains hydrostatic pressure on the subterranean formations through which the well bore is drilled to thereby prevent pressurized formation fluids from entering the well bore and circulates cuttings out of the well bore.
Once the well bore has been drilled to the desired depth, a string of pipe referred to as casing is positioned in the well bore. A hydraulic cement composition is pumped into the annular space between the walls of the well bore and the casing and allowed to set thereby forming an annular sheath of hardened substantially impermeable cement in the annulus. The cement sheath physically supports and positions the casing in the well bore and bonds the casing to the walls of the well bore whereby undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
The subterranean formations into or through which well bores are drilled often contain naturally occurring or drilling induced weak zones having low tensile strengths and/or openings such as fractures, faults and high permeability streaks through which drilling fluid is lost from the well bores or pressurized formation fluids enter the well bores. The weak zones in the well bore have low pressure containment integrity and are subject to failure as a result of the hydrostatic pressure exerted thereon by drilling fluid or other treating fluid such as hydraulic cement slurries. That is, when a well fluid such as drilling fluid or a hydraulic cement slurry is introduced into the well bore, the combination of hydrostatic and friction pressure exerted on the walls of the well bore can exceed the strength of weak zones in the well bore and cause well bore fluid outflows into the formation containing the well bore. When the formation contains induced or natural formation fractures, faults, or the like, well bore fluid outflows and/or pressurized formation fluid inflows, or both, can take place. The inflows and/or outflows make the well unstable. When a well becomes unstable, major problems such as lost circulation and blow-outs can occur which require the drilling operation to be terminated and costly remedial steps to be taken.
By way of further example, formation sands and shales can be encountered while drilling having unexpected low pressure containment integrity. Thus, at any depth during the drilling or completion of a well bore, the well bore fluid circulating densities and pressures can exceed planned or designed densities and pressures. The excess pressure exerted within the well bore can and often does exceed the subterranean formation pressure containment integrity which causes loss of well bore fluids into the formation. Such loss can lower fluid column heights in the well bore, reduce hydrostatic pressure below formation pore pressures and cause pressurized formation fluid inflow. When this happens, rig operators are often forced to prematurely set casing or run a drilling liner in the well bore making the overall cost of the well much higher than expected.
Thus, there are needs for reliable and quick methods of discovering, diagnosing and correcting formation integrity problems in well bores during drilling.